Apparatus for powder coating metal articles

ABSTRACT

A method and apparatus for powdercoating surfaces of articles selectively whereby coating is obtained only on the desired surfaces while leaving other surfaces uncoated are provided. The invention is particularly adapted for the coating of container ends with powdered materials by applying the material selectively thereto and permitting recovery of the excessive material which is recovered and recycled to the system.

United States Patent Blakeslee 1451 Nov. 11, 1975 1 APPARATUS FOR POWDER COATING METAL ARTICLES [75] Inventor: Harry Nelson Blakeslee, Barrington.

American Can Company, Greenwich. Conn.

Filed: Apr. 17, 1974 Appl. No.: 461,696

Assignee:

US. Cl. 118/630; 118/312; 118/301; 118/326; 118/504; 427/30 lnt. C1. B0513 5/02; B05B 7/14 Field of Search 118/630, 629, 627, 602. 118/301, 312. 326, 504

References Cited UNITED STATES PATENTS 7/1952 Paasche 118/312 11/1956 Marantz 118/312 9/1961 Jackson et a1. 118/627 3.2711451 10/1966 01111111 61111 1111/31: 3,623,854 11/1971 Frank 1111/3111 3.694.242 9/1972 Ofner... 118/627 3.714.926 2/1973 Ofner 118/312 3.866.567 2/1975 Fritzschz 1111/3114 Primary E.\am1'1zerMervin Stein Assistant Evan1i/1e1'-Douglas Salser Attorney, Agent. 01' Fi1'111Robert P. Auber; Ernestine C. Bartlett; George P. Zichmer [57] ABSTRACT A method and apparatus for powdercoating surfaces of articles selectively whereby coating is obtained only on the desired surfaces while leaving other surfaces uneoated are provided. The invention is particularly adapted for the coating of container ends with powdered materials by applying the material selectively thereto and permitting recovery of the excessive material which is recovered and recycled to the system.

9 Claims, 1 Drawing Figure APPARATUS FOR POWDER COATING ARTICLES BACKGROUND OF THE INVENTION Many methods are known for providing protective 5 coatings on articles. These existing methods, however, suffer from several disadvantages. For example, in conventional lacquer spray applications, in many cases, uniform coverage is difficult to obtain, especially on irregular shapes. Significantly, control of the emission of solvents in such processes is an ever increasing problem.

Another commonly used method is that of powder spraying. In this method, powdered material is sprayed and thermoadhesively applied to articles preheated to the fusion temperature of the powdered material or, alternatively the particlesare sprayed through an electrostatic field whereby the powdered material adheres to the article by electrostatic charge. Most of such procedures involve moving articles in and out of spray chambers and are accompanied by dust problems, often with waste of large amounts of the powdered material. Additionally, it is often difficult to obtain desirable film thickness on coated parts by these types of spraying. With many of such techniques to obtain adequate thickness it will be required that large amounts of the powdered material be held in position adjacent the article for a substantial period of time. This requirement often precludes obtaining speeds suitable for commercial production. Moreover, it is often difficult to selectively coat precise areas of an article and, for this reason, most coventional powder spray coating techniques are limited to continuous coating of unmasked items. When masking is employed, it is usually accomplished by application of tape or a stencil or other physical masking fixture directly to the areas of the article that is not desired to coat. ln general, a masking fixture will be sufficiently conductive so that some of the powder will adhere thereto during spraying. If the fixture is removed prior to baking, the power adjacent thereto will be disturbed and lead to a poor finish. If the fixture is left on during the baking of the coating, the coating adheres to the fixture making it difficult to remove without adversely affecting the coating. Thus, while powder coating has certain advantages over other coating techniques, the problems arising in obtaining coatings of adequate thickness and uniformity, of operating at commercial speeds, of controlling dust and waste of material and of obtaining adequate masking where selective coatings are desired present substantial problems in the spraying method.

Several proposals have been made to obviate many of the problems above enumerated. For example, US. Pat. No. 3,714,926 issued Jan. 6, 1973 to Ofner provides an apparatus for electrostatically coating surfaces with powdered particles by spraying the material in an air current and recovering the excessive material which is drawn off by suction from the spray chamber, treated and returned to a supply for reuse. Thus, while waste of material and dust would not appear to be severe problems in such an operation, the proposal still does not provide a solution to the problems associated with selectively coating specific areas of articles and the ingress and egress of articles through chambers.

This invention provides a method and apparatus whereby all of the disadvantages of prior powder coating techniques enumerated above are obviated. The in- METAL vention permits rapid and selective coating of articles, obtaining films of desired thickness whereby coating is obtained only on the desired surfaces, dust problems and waste of materials are eliminated and recovery and reuse of excessive material is realized.

More specifically, the invention provides a method of selectively coating areas of an article with powdered material which comprises the steps of:

admixing powdered material with an air stream and conveying the material to a spray chamber and preferably electrically charging said powdered material prior to conveying it to said spray chamber;

spraying the powdered material from said chamber upon an article to be coated positioned outside said chamber, said coating being applied to predetermined areas of said article while other areas of the article are masked to remain free from contact with particles, said masking being accomplished by an annular vacuum port disposed about the orifice of said spray chamber which shields portions of said article from the powder and withdraws excess material not adhering to said article; and recovering said withdrawn excess material. The method also preferably includes admixing the withdrawn material with new powdered materials;

electrically charging said mixed material, if desiried. supplying the material to the spray chamber in an air stream;

and spraying said material from said chamber upon another article to be coated.

The above method is accomplished preferably employing an apparatus of the invention which comprises, in combination:

A. A spray device comprising a spray chamber and having 1. means for injecting air and conveying powder particles to a spray chamber;

2. optionally and preferably, means for electrically charging said particles;

3. means for directing the flow of said air stream of particles upon an article to be coated positioned outside the orifice of said spray chamber; and

B. A mask chamber comprising an air stream disposed about the orifice of said spray chamber for shielding predetermined areas of said article from contact with said particles.

In a most preferred embodiment of the invention, the apparatus also additionally comprises:

C. A vacuum manifold connected to said spray chamber and to said mask chamber through a first low pressure air passage and a second higher pressure passage, respectively, for withdrawing an air stream containing excess powered material;

D. A powder supply'chamber connected to said spray device through apowder feed passage and through said vacuum manifold having 1. means for supplying new powdered material for admixture with withdrawn powdered material; 2. means for reconditioning said powdered material; and 3. means for feeding powdered material to said spray device; and

E. a blower creatingan air current which exhausts air from each of said spray chamber, mask chamber, vacuum manifold and power supply chamber.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawing is a schematic cross- 3 sectional view of a preferred apparatus of the inven tion. a

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawing, there is shown a container 1 supported on acontainer indexing unit (not shown), comprising, for example, a turret 2 to which a timer is connected through a proximity sensor 4. The container is positioned at one end of a spray device 8 which rests on a suitable supporting base (not shown) comprising a spray chamber 12 and air passages 14, 16 and 18. A mask chamber 20 comprising an air passage 22 is disposed peripherally around the orifice of the spray chamber. Air passages 18 and 22 are connected to a vacuum manifold 24 which is connected through a suitable connecing line or overspray tube 26 to a powder supply chamber 28. Asupply of fresh powder is contained in a suitable bin 30 and is connected to the supply chamber through line 32. The chamber comprises filters 34, preferably at its upper end, a rotary valve 36 at its lower end, a reconditioner 38 and is connected to the spray device through powder feed passage 40. A blower 42 is also connected at a point opposite or above the filters of the powder supply chamber.

As shown in the drawing, powder is fed to the spray device by gravity through the powder feed passage 40. Air pressure regulators l and 17 and solenoid valves 19 and 21 which function to introduce and permit control of air injected to fluidize and aspirate the powder, respectively, are arranged in close proximity to the spray chamber 12 to minimize powder transport times and to maximize timing accuracy of the powder flow in high speed operation. High pressure air, from a suitable source, is directed to two paths through the regulators and solenoid valves with and 19 furnishing air to fluidize the powder feed and 17 and 21 introducing the injection air to the aspirator 9 which blows the powder onto the container. The solenoid valves are preferably pulsed simultaneously for the appropriate length of time to coat the container end while it is in-station. The fluidizing air is preferably also pulsed rather than continously fed. In typical operation, the injection air pressure may vary from about 5 to 90 psi while the fluidizing air pressure may vary from about 0 to 50 psi. Varying either of these pressures will change the film thickness. These regulators therefore offer ready control of the film thicknesses obtained. It has been found that increasing the injection air pressure increases the film thickness while increasing the fluidizing air pressure tends to reduce the film thickness. In practice, it is con- .venient to operate at a constant fluidizing pressure and vary only the injection air pressure thereby producing films varying from above five mils to less than one mil, as desired. Optimumresults have been obtained with electrostatically charged particles employing injection air pressure at about 30 psi and fluidizing air pressure at about 6 psi.

Where the particles are not electrically charged, a fluidizing pressure of about 0.5 psi and injection pressure of about 60 psi have givensatisfactory results. Following fluidization of the powder as described above, the injection air carries the' stream of particles into the aspirator 9 where it preferably travels past means for electrically charging the same. Any charging means, such as corona discharge, arcs, sparks orradiationmay be used. In the embodiment shown, high voltage electrodes 10 are employed which serve to electrically charge the particles which flow through the spray chamber 12 and mask chamber 20 to the container end. Optionally and preferably, the spray chamber of the spray device contains vent openings 11 to permit expansion of the pressurized air stream. Disposed within the chamber is a diffusion cone 13 which may be of suitable configuration, for example, a disk or cone of suitable material, preferably plastic, to direct and aid incontrol of the flow of the particles to the container end. The cone may be mounted on a rod coaxial with the aspirator or independently mounted on supports attached to the side walls of the spray chamber. The diffusion cone, an optional feature of the present apparatus, is preferred, since it has been found that the powder stream emanating from the aspirator and applied to the article has a tendency to be more concentrated in the center portion of the can end than the outer portion where it is most desired. Placement of the cone tends to even the powder distribution.

The spray chamber 12 and mask chamber 20 are connected to air passages 18 and 22, respectively, which are jointly converged in the vacuum manifold 24. In operation, air at atmospheric pressure enters the mask chamber around the periphery of the container end to mask and prevent powder from contacting the sidewall of the container. The vacuum in the respective chambers may be controlled through any means desirable, for example employing butterfly valves or by decreasing the diameter of one passage relative to the other. The vacuum is greater in the mask chamber than in the spray chamber. The lesser vacuum in the spray chamber precludes disturbance of the powder spray pattern, allowing an optimum amount of particles to reach the article and yet is sufficient to withdraw excess material that falls into the passage. The greater vacuum in the mask chamber is sufficient to withdraw atmospheric air entering around the periphery of the container end and the spray chamber orifice and to withdraw excess powder thereby preventing powder particles from contacting the sidewalls of the container. Specific pressures are not critical as long as the difference in the air pressures in the respective chambers is sufficient to enable the apparatus to function as described above. In operation, for examples, vacuums as low as 1 inch of water in the mask chamber and onefourth inch of water in the spray chamber have been found to be suitable.

Excess powder withdrawn through the spray and mask chambers is converged in the vacuum manifold and is fed through the overspray line 26 to the powder supply chamber 28 by vacuum powdered on the total system by the blower 42. Overspray powder is filtered through suitable means such as bag filters 34, from which particles are separated from the air current created by the blower and which are periodically cleaned by air pulsed to their inner surfaces. The powder falls to the lower portion of the unit, which is preferably cone shaped, into the rotary valve 36. The system is replenished with fresh powder as needed through a tube 32 connected through valve 31 to the bin of fresh powder 30. The rotary valve has a plurality of compartments into which powder falls and is driven by suitable means such as an electric motor so that with each revolution a predetermined amount of powder from the container will fall into the reconditioner 38 which may be simply a rotary brush revolving against a screen. The

treated material is then fed to the spraydevice bygravity in readiness forthe' next coating procedure.

The powders employed maybe any material capable of being electrostatically charged and/ortherm'ally adhered and sprayed. in general such materials afe thermoplastic or therrnosetting resins including polyester, vinyl, polyamide, polyethylene and epoxy resins and mixtures thereof. Especially preferred are expoxy resins such as the glycidyl ethers of bisphenol Ax'I-he pow- I dered materials may include suitable hardeners, pigments and fillers as. desired. 1

In general, the powders are prepared by grinding the bulk materials. In practicing the instant invention, the powder particles are preferably'ground to have particles within the range of about 5 to 150 microns with an average particle size of about 30 to 75 microns. Most preferably the powder will have an average particle size of microns or less although the ready adjustment of fluidizing and injection pressures in the spray device makes the process operable with generally any particle size range.

As indicated hereinabove, the apparatus of the invention is effective for electrostatic powder coating or thermo-adhesive powder coating procedures. In the fonner case, the container may be preheated, if desired, although satisfactory results are obtained in the absense of such preheating step.

When applying the particles thermoadhesively, it is necessary that the containers be preheated to an elevated temperature that is sufficient to cure'the resin particles to the article but insufficient to cure the resin upon contacting the article since flow out of the coating during subsequent curing is desired to obtain satisfactory coatings in terms of rheological and physical properties. Adequate coatings have been prepared herein utilizing temperatures of about 275F to about 350F, with higher or lower temperatures being operable depending upon the particular resin employed.

The mode of operation of the apparatus according to the invention which may be employed for electrostatically powder coating a container end with an epoxy resin is as follows.

A powdered formulation comprising an epoxy resin (for example, a glycidyl ether of bisphenol A available commercially as DER-642, Dow Chemical Comapany), a suitable catalyst (for example dicyandiamide) and pigment (for example TiO is charged to the powder supply chamber from the fresh powder bin where it is fed through the rotary valve, reconditioner and powder feed passage to the spray device. A container is indexed to the orifice of the spray chamber and positioned so that the bottom end may be coated.

A proximity sensor and timer trigger the solenoid valves and air regulators injecting air to fluidize the powder feed at about 6 psi and to the aspirator at about 30 psi. The fluidized particles in the air stream are sprayed past the electrodes where the particles are charged and directed against the container end. The particles adhere electrostatically and uniformly to the surface of the container end. At the same time, the side walls of the container are shielded from contact with the particles by the mask chamber, excess particles are withdrawn through the vacuum manifold and returned -to the powder supply through the overspray where they are filtered from the airstream, mixed with new powder, reconditioned and fed to the spray device. The

coated container is then conveyed to a curing oven and jhea atjelevated temperaturewhereby the powder is converted to 'a hard, fused coating of approximately 1 ,mil thicknessoverthe container endl Anew container isijnd exed to the spray "device andthe' process is re- 5 "peated. j L. Comparabler'esults are obtained when powder coat- "ing a container end but in which the particles are thermoadher'ed' when the above example is repeated except that the highvoltage electrodes are disconnected, .the [container is preheated t0.?b0llt 300F, the fluidizing air pressure is about 0.5 psi and the pressure of air injected totheaspirator is about 60 psi. a Although the invention has been illustrated with 'metal, containers, itwwill be understood that the apparatus and'method arc applicableto the coating of any article where it is desired to selectively coat portions only of ,the surfaces while leaving other areas uncoated. It will be seen that optimum use of powdered material is realized with little or no waste thereof since the only powder that leaves the system is that applied to the article. Additionally, the method is capable of being carried out at very high speeds that are eminently suitable for commercial operation.

It is thought that the invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the structure of the apparatus and in the steps of the method and their order of accomplishment without departing from the spirit and scope of the invention or sacrificing all of its attendant advantages. the method and apparatus described being merely preferred embodiments thereof.

What is claimed is:

1. An apparatus for selectively powder coating areas of an article which comprises, in combination:

A. a spray device comprising a spray chamber and having 1. means for injecting air and conveying powder particles in an air stream to a spray chamber;

2. means for spraying the charged particles upon an article to be coated positioned outside an orifice of said spray chamber; and

B. a mask chamber comprising an air current disposed between the orifice of the spray chamber and the article for shielding predetermined areas of said article from contact with said particles.

2. An apparatus as claimed in claim 1 additionally comprising:

a vacuum manifold connected to said spray chamber and to said mask chamber through a first low pressure air passage and a second higher pressure air passage, respectively, for withdrawing air and excess powdered material;

a supply chamber connected to said spray device and said vacuum manifold comprising means for feeding powdered material to said spray device; and

a blower creating an air current which exhausts air from said spray device, mask chamber, vacuum manifold and powder supply chamber.

3. An apparatus as claimed in claim 1 wherein sai spray device also comprises means for electrically charging said particles.

4. An apparatus as claimed in claim 1 wherein said spray device also comprises means for directing the flow of the particles upon said article.

5. An apparatus as claimed in claim 4 wherein said means is a diffusion cone.

6. An apparatus as claimed in claim 2 wherein said power supply chamber also comprises means for supplying new powdered material.

7. An apparatus as claimed in claim 2 wherein said powdered material withdrawn by said vacuum manifold is conveyed to said powder supply chamber.

8. An apparatus as claimed in claim 6 in which said powde'r supply chamber also comprises means for reconditioning said withdrawn material.

9. An apparatus for electrostatically and selectively coating ends of a metal container which comprises, in combination:

A. a spray device comprising a spray chamber and having means for electrically charging and spraying particles of powder upon the end of a container positioned at the orifice of said chamber;

B. a mask chamber having an air current disposed between the orifice and the container which shields the sidewalls of the container from contact with said particles; C. a vacuum manifold connected to said spray chamber through a low pressure vacuum port and to said mask chamber through a high pressure vacuum port for withdrawing air and excess powdered material; D. a powder supply chamber connected to said spray device and said vacuum manifold comprising 1. means for supplying new powdered material for admixture with withdrawn material;

2. means for reconditioning said powdered material 3. means for feeding reconditioned powdered material to said spray device and E. a blower creating an air current which exhausts air from each of said spray chamber, mask chamber,

vacuum manifold and powder supply chamber. =l 

1. means for supplying new powdered material for admixture with withdrawn material;
 1. means for injecting air and conveying powder particles in an air stream to a spray chamber;
 1. An apparatus for selectively powder coating areas of an article which comprises, in combInation: A. a spray device comprising a spray chamber and having
 2. An apparatus as claimed in claim 1 additionally comprising: a vacuum manifold connected to said spray chamber and to said mask chamber through a first low pressure air passage and a second higher pressure air passage, respectively, for withdrawing air and excess powdered material; a supply chamber connected to said spray device and said vacuum manifold comprising means for feeding powdered material to said spray device; and a blower creating an air current which exhausts air from said spray device, mask chamber, vacuum manifold and powder supply chamber.
 2. means for spraying the charged particles upon an article to be coated positioned outside an orifice of said spray chamber; and B. a mask chamber comprising an air current disposed between the orifice of the spray chamber and the article for shielding predetermined areas of said article from contact with said particles.
 2. means for reconditioning said powdered material
 3. means for feeding reconditioned powdered material to said spray device and E. a blower creating an air current which exhausts air from each of said spray chamber, mask chamber, vacuum manifold and powder supply chamber.
 3. An apparatus as claimed in claim 1 wherein said spray device also comprises means for electrically charging said particles.
 4. An apparatus as claimed in claim 1 wherein said spray device also comprises means for directing the flow of the particles upon said article.
 5. An apparatus as claimed in claim 4 wherein said means is a diffusion cone.
 6. An apparatus as claimed in claim 2 wherein said power supply chamber also comprises means for supplying new powdered material.
 7. An apparatus as claimed in claim 2 wherein said powdered material withdrawn by said vacuum manifold is conveyed to said powder supply chamber.
 8. An apparatus as claimed in claim 6 in which said powder supply chamber also comprises means for reconditioning said withdrawn material.
 9. An apparatus for electrostatically and selectively coating ends of a metal container which comprises, in combination: A. a spray device comprising a spray chamber and having means for electrically charging and spraying particles of powder upon the end of a container positioned at the orifice of said chamber; B. a mask chamber having an air current disposed between the orifice and the container which shields the sidewalls of the container from contact with said particles; C. a vacuum manifold connected to said spray chamber through a low pressure vacuum port and to said mask chamber through a high pressure vacuum port for withdrawing air and excess powdered material; D. a powder supply chamber connected to said spray device and said vacuum manifold comprising
 1. means for supplying new powdered material for admixture with withdrawn material;
 2. means for reconditioning said powdered material
 3. means for feeding reconditioned powdered material to said spray device and E. a blower creating an air current which exhausts air from each of said spray chamber, mask chamber, vacuum manifold and powder supply chamber. 